1. Field of the Invention
The present invention relates to drive systems for electric motors and, more particularly, to a drive system for a permanently excited electric motor.
2. Description of the Related Art
Permanently excited electric motors are used, for example, as vehicle drive motors which, in the case of hybrid drive systems, receive electrical power from a generator driven by an internal combustion engine or, in the case of completely electrically operated vehicles, receive electrical power from a so-called traction battery. In the case of permanently excited electric motors, and due to their construction, there is, in principle, the problem that, on account of the relative movement which occurs during operation between the armature windings and the permanent magnets, a back e.m.f., known as the field e.m.f., is induced in the armature windings. This induced voltage rises as the speed increases, until it ultimately lies in the range of the supply voltage provided by the voltage source for the electric motor. A further increase in the motor speed can then be obtained only by means of so-called field weakening, in which a phase-winding shift in the current introduced into the windings is produced. If faults occur in such drive systems with permanently excited electric motors, such as the failure of the drive unit performing the field weakening, this may lead to serious problems. As can be taken from characteristic curve A in FIG. 3, which reproduces the braking torque for a permanently excited electric motor in the case of passive regeneration by the rotating electric motor, a failure of the field weakening, in particular at high speeds, leads to a considerable braking torque, which is produced when electrical energy is fed back from the motor into the voltage source, for example the traction battery. Due to the voltage overshoot caused by the internal resistance of the battery, there is the risk that damage will occur in the area of the traction battery, which is generally designed to output an operating voltage of about 200 to 300 V. In addition, the occurrence of such large braking torques during driving operation is undesirable, since this may result in endangering a person seated in a vehicle.
If no voltage source is connected to the intermediate circuit, or if, for example, the connection between the voltage source and the intermediate circuit is broken, the field e.m.f. may be present on the intermediate circuit. This can lead to damage in the area of the components present in the intermediate circuit or the power electronics for the electric motor and/or possibly in a generator, in particular semiconductor components.
In order to avoid these problems, the various components of the converter, for example the capacitors and power semiconductors, have been designed such that even the rectified field e.m.f. does not exceed their rated voltage, so that damage to these components is not expected. The consequence of this is that the drive systems could not be designed such that they are able to produce the greatest possible output in the volume available to them and a certain oversizing of the components always had to be provided. In addition, the various components in the intermediate circuit or the power electronics had to be designed for the maximum voltages to be expected, so that higher costs arose in the area of these components.